Your flush rule is correct.
Hanna meters suck furry monkey balls. I've seen people kill their plants trusting those worthless pieces of CRAP. I'd trust the liquid better in general, and cross-check on the regular.
ProMix is appropriate where your plants will be repotted approximately once per month into fresh promix.
After teh flush, feed with a 1/4 strength complete fertilizer. You may also do a foliar feed of the same to help jump-start the deficiency correction.